The present invention relates to a heat-fixing apparatus adapted to be mounted in image-forming apparatuses such as electrophotographic apparatuses and electrostatic printing apparatuses for fixing a toner image formed on a paper or other sheet member between a pair of rolls rotating at some contact pressure, at least one of which is provided with a heating means, and more particularly to an improved heat-fixing apparatus capable of fixing a toner image even on a toner image-bearing sheet member consisting of a plurality of plies such as an envelope without suffering from wrinkling or creasing.
Conventionally known as a fixing apparatus for use in image-forming apparatuses is a heat-fixing apparatus of a heating roll type comprising a heating roll and a pressure roll arranged opposite to each other, the heating roll being constituted by a metal cylinder having good thermal conductivity coated with a non-adherent, heat-resistant layer and containing a heat source such as an infrared lamp, a halogen lamp or a nickel-chromium wire therein, and the pressure roll being constituted by a metal cylinder coated with a heat-resistant, resilient surface layer. In the above heat-fixing apparatus, the fixation of a toner image formed on a sheet is carried out by energizing a heat source to heat the surface of the heating roll to temperatures necessary for fixing of the toner, rotating the heating roll and the pressure roll at a proper contact pressure, and passing the sheet bearing the toner image between the two rolls.
In such a heat-fixing apparatus, it is particularly important to prevent offset and wrinkling of the image-bearing sheet in order to obtain a high-quality fixed image.
Conventionally used to prevent offset of the fixed image is a heating roll coated with resins having good release properties such as polytetrafluoroethylene [PTFE], perfluoroalkoxy-tetrafluoroethylene copolymers [PFA], etc. Such a heating roll, however, has insufficient fixing capability because of little resilience, and also is susceptible to surface damage. To eliminate these defects, a proposal was made to provide a heating roll coated with a layer of a mixture of a fluorine rubber and a fluorine resin, which is baked to have a fluorine resin layer as an outermost layer (U.S. Pat. No. 4,568,275). This heating roll, however, is poor in durability. Specifically speaking, because it has a fluorine resin surface layer of only several .mu.m in thickness, the surface layer is worn out by producing only several tens of thousand copies due to contact with a cleaning member, etc.
Thus, a proposal was made to provide a heating roll having a fluorine resin layer over the above mixture layer (Japanese Patent Laid-Open No. 59-217010).
On the other hand, with respect to the prevention of wrinkling or creasing of paper sheets, various proposals were made. For instance, Japanese Utility Model Laid-Open No. 54-98226 discloses a heat-fixing apparatus comprising a heating roll and a pressure roll contacted with each other at low pressure, the pressure deformation ratio of both rolls being identical or nearly equal so that it can prevent meandering and creasing of even thick copy papers. And Japanese Patent Laid-Open No. 59-74578 discloses the use of a heating roll and a pressure roll, each of which is constituted by a core cylinder, a heat-resistant, resilient coating layer formed thereon, and a fluorine resin layer coated on the above layer to prevent creasing of copy papers even under severe conditions such as producing copies on both sides of thin paper sheets. Recently, investigations have been carried regarding the feasibility of printing addresses etc. directly on the envelopes using copier apparatus. The problem encountered in this printing process is that the envelopes bearing a toner image are susceptible to creasing when supplied to the above heat-fixing apparatus. Once creases appear on an envelope, it is not only bent or deformed, but also the printed address is destroyed or deformed by creases, rendering it unreadable in extreme cases. This is because a sheet member constituting an envelope is naturally overlapped, and its glued portion near the opening or its overlapped portion is constituted by nearly three or four plies of sheets, thereby increasing the maximum thickness of the envelope and causing the sliding of two overlapping sheets. This kind of problem has never been experienced by usual copiers. To cope with this problem, lower fixing pressure for fixing an image on an envelope than the fixing pressure for usual plain papers has been proposed (Japanese Patent Laid-Open No. 61-294475).
However, in the case of fixing a toner image on an envelope, the above-mentioned conventional techniques cannot provide satisfactory results, namely unable to prevent creasing of a paper sheet while retaining good fixability. Specifically speaking, according to Japanese Patent Laid-Open No. 61-294475, good fixation of a toner image cannot be achieved because of low fixing pressure, though creasing can be prevented. And Japanese Utility Model Laid-Open No. 54-98226 is unable to provide sufficient fixability because of the structure that both rolls are contacted with each other at small pressure to maintain equal or nearly equal the deformations of both rolls in contact portions. Further, the apparatus of Japanese Patent Laid-Open No. 59-74548 cannot be expected to prevent creasing under such severe conditions as fixing a toner image on an envelope, because the heating roll and the pressure roll have resilient layers of different thickness, and are unable to form a substantially flat nip portion therebetween.